A technique that uses an inkjet print head to preform printing is known for printing identification information such as characters, letters, marks or the like on a tablet. In a tablet printing apparatus using this technique, tablets are conveyed by a tablet conveying device such as a conveyor. Ink (for example, edible ink) is ejected from a nozzle of the inkjet print head located above the tablet conveying device toward each tablet passing under the print head to print identification information on the tablet. As the tablet conveying device, there has been developed a device that holds tablets on the conveyor belt with a suction force. This tablet conveying device sucks the tablets onto the conveyor belt to hold them.
Generally, a plurality of, for example, circular or rectangular suction ports are formed in the conveyor belt so as to line up in the conveying direction to suck and hold tablets. The tablets supplied onto the suction ports are held on the conveyor belt by suction from the suction ports. However, there may be a suction port that is not completely closed by a tablet. For example, the suction port may be closed only partly depending on the size, shape, posture, or the like of the tablet, or the suction port may not be closed at all due to random supply of the tablets. When the suction port is not completely closed by the tablet, an airflow is generated above the suction port as the air is sucked from the suction port. In particular, when the suction force is strong, the amount and the flow rate of air sucked increase. This results in a stronger airflow or increased reach of the airflow, and thereby turbulence may occur.
With the inkjet print head, printing is performed by ejecting ink from the print head toward the tablet such that the ink hits on the surface of the tablet. Accordingly, the ink ejected from the print head is flying between the print head and the tablet until it hits on the tablet. At this time, if an airflow is generated in the space between the print head and the tablet, the ink does not hit on a desired position, and the print quality is degraded. For example, in the case where the airflow is strong, the airflow has a wide reach, or the airflow is disturbed, the print quality is remarkably degraded. Further, when the influence of the airflow reaches the vicinity of the nozzle of the print head, the ink around the nozzle dries, thereby causing ejection failure. As a result, the print quality is also degraded. In addition, ink that has not hit on the tablet may sometimes scatter in the form of mist. If the ink scatters like mist, for example, it may adhere to the side surface of the tablets being conveyed.
Therefore, in order to suppress the airflow, it is necessary to reduce the suction force of the suction port, i.e., the suction force for sucking the tablets. However, if the suction force for sucking the tablets is reduced, the force for supporting the conveyor belt is also reduced since the conveyor belt is sucked and supported through the same path as the suction path for sucking the tablets. When the force for supporting the conveyor belt is reduced, the conveyor belt may be lifted off the conveyor body or it may move up and down. In this case, the positional deviation of the tablet occurs, and the print quality is degraded.